To clarify the apparent confusion arising from the diversity of published stability statistics, and the relationship of these with the clustering of genotypes for similarity of response to environments, the interrelationship of nine stability statistics and nine similarity measures are investigated. The stability statistics fall into four groups depending on whether they are based on the deviations from the average genotype effect or on the genotype X environment (GE) term, and whether or not they incorporate a regression model on an environmental index. These groups of stability statistics are shown to be related to three concepts: A genotype may be considered to be stable (i) if its among-environment variance is small, (ii) if its response to environments is parallel to the mean response of all genotypes in the trial, or (iii) if the residual mean square from a regression model on the environmental index is small. Unfortunately, these three concepts represent different aspects of stability and do not always provide a complete picture of the response. In the alternative approach of cluster analysis, the similarity measures define complete similarity in three different ways: i) equality of genotype's response across locations, ii) equality of all within location differences, and iii) equality of all within location ratios. The advantage of the nonparametric approach is that a cultivar's response characteristics can be assessed qualitatively, without the need for a mathematical characterization.

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A 15-month field experiment was performed in the emergent zone of a freshwater riverine marsh along the Ottawa River, Canada, to determine whether the mechanisms producing plant zonation along the exposure gradient of freshwater shorelines also accounted for the zonal pattern along the water depth gradient. Three species (Carex crinita, Acorus calamus, and Typha angustifolia) were chosen, having contiguous distributions along a gradient of water depth. Ramets of each were planted within and beyond the field distributions of each, both in the presence and in the absence of the natural vegetation of each site. Although there was strong evidence of growth depression due to the presence of neighbouring plants (interference), there was no evidence of a differential response between species, between sites on the water depth gradient, or a combination of the two. As well, thee was no evidence that the water depth gradient represents a general gradient of decreasing productivity; rather, there was a qualitative change below the low-water level. These results are contrary to previously published results obtained along the exposure gradient of freshwater shorelines, where the effects of plant interference do vary both along the exposure gradient and among species. Key words: plant zonation, interference, water depth, exposure gradient.